1. Field of the Invention
The present invention generally relates to a four-cycle internal combustion engine and, more particularly, to the four-cycle internal combustion engine for use as a power plant in a small size, portable working machine such as, for example, a bush cutter and a mowing machine.
2. Description of the Related Art
It is generally known that the small size, portable working machine such as, for example, a bush cutter is operated adaptively in all positions considered suitable, necessary, optimal and/or convenient for the operator to perform an intended work. Accordingly, the small size, portable working machine has long employed a two-cycle combustion engine of a type utilizing an oil-mixed fuel, i.e., a mixture of fuel with oil. However, in recent years, in view of the pressing demand to substantially purify exhaust gases emitted from the working machine, a four-cycle combustion engine capable of being used as a power plant in the small size, portable working machine has come to be developed such as disclosed in, for example, the Japanese Laid-open Utility Model Publication No. 4-93707.
The four-cycle combustion engine suggested in the above mentioned publication includes an oil sump disposed at the bottom of the crankcase, the design of which is specifically tailored to prevent lubricant oil, accumulated within the oil sump, from leaking even when the portable working machine is operated in any position inclined within a predetermined angle. However, with this four-cycle combustion engine, it has been found that since the lubricant oil within the oil sump tends to flow into a combustion chamber particularly when the portable working machine is so inclined as to assume a substantially inverted (i.e., upside down) position, such portable working machine is in effect incapable of being used in all positions. Also, considering that a substantially large amount of lubricant oil is accommodated within the oil sump, the portable working machine as a whole tends to be so heavy as to impose an increased amount of labor on the operator.
On the other hand, the four-cycle combustion engine requiring no oil sump and capable of being used in all positions is suggested and disclosed in, for example, the Japanese Laid-open Patent Publication no. 8-100621. With the four-cycle combustion engine disclosed in this second mentioned publication, an air-fuel mixture containing oil similar to that used in the two-cycle combustion engine is utilized and is introduced into the crankcase. The air-fuel mixture within the crankcase is, by the utilization of change in pressure inside the crankcase that occurs as a result of a reciprocating motion of the piston, allowed to flow through a first air-fuel passage communicated directly with an intake port as well as through a second air-fuel passage communicated with the intake port through a valve operating mechanism, into the combustion chamber through the intake port, so that a lubricant oil contained in the air-fuel mixture can be utilized to lubricate various parts within the crankcase and those of the valve operating mechanism.
In the four-cycle combustion engine that can be used in all positions such as disclosed in the second mentioned publication, during the intake stroke the air-fuel mixture containing oil within the crankcase that is compressed as a result of a descending motion of the piston is supplied to the intake port through the first air-fuel passage. On the other hand, the air-fuel mixture is also supplied through the second air-fuel passage into a valve chamber of the valve operating mechanism and a portion of the air-fuel mixture introduced into the valve chamber is subsequently introduced into the intake port through a small opening defined in the bottom of the valve chamber and defining a part of the breathing passage. At this time, the lubricant oil then pooled within the valve chamber flows outwardly from the valve operating mechanism through the opening at the bottom of the valve chamber and into the combustion chamber through the intake port. This results in white fume generated undesirably.
Also, in the four-cycle combustion engine disclosed in the second mentioned publication, since the opening at the bottom of the valving chamber is too small for the air-fuel mixture to flow smoothly within the valve chamber, the lubricant oil contained in the air-fuel mixture tends to stick to wall surfaces of passages where the air-fuel mixture flows little and, therefore, various parts of the engine will hardly be lubricated effectively.